The subcellular distribution of acid phosphatase in malignant keratinocytes of invasive laryngeal carcinoma was studied by ultrastructureal cytochemistry. The reaction product was localized in Golgi, ER, and some cytoplasmic vesicles. Acid phosphatase activity was also observed in lysosomal structures in the cortical cytoplasm of basal carcinoma cells. Extracellular acid phohphatase activity also occurred at the tumor-stroma junction in membrane-bound vesicular structures. The localization of acid phosphatase in this report is discussed in relation to acid phosphatase activity in other tumors. The findings lend further support to the important role of hydrolytic enzyme release in respect to tumor invasion into surrounding tissues.